NISTHY.
This kind of attraction is evinced by a variety of familiar experiments ; as, by the union of two contiguous drops of mercury ; by the mutual approach of two pieces of cork floating near each other in a basin of water ; by the adhesion of two leaden balls, whose surfaces are scraped and joined together with a gentle twist, which is so considerable, that if the sur faces are about a quarter of an inch in diameter, they will not be separated by a weight of 100M ; by the ascent of oil or water between two glass planes, so as to form the hyperbolic curve, when they are made to touch on one side, and kept separate at a small distance on the other ; by the depression of mercury, and by the rise of water in capillary tubes, and on the sides of glass vessels ; also in sugar, sponge, and all porous substances. And where this cohesive attraction ends, a power of repulsion begins.
It is uncertain in what proportion this force decreases as the distance increases. Desaguliers conjectures, from some phe nomena, that it decreases as the biqua iratic or 4th power of the distance, so that at twice the distance it acts 16 timer more weakly, &c.
To determine the force of cohesion, in a variety of different substances, many experiments have been made, and parti cularly by professor Muschenbroek. The adhesion of polished planes, about two inches in diameter, heated in boiling water, and smeared with grease, requi red the following weights to separate them.
Cold Grease. Hot Grease.
lb. lb.
Planes of Glass . . 130 . . 300 Brass . . 150 . . 800 Copper . 200 . . 850 Marble . . 225 . . 600
Silver . . 150 . 250 Iron . . . 300 . . 950 But when the brass planes were made to adhere by other sorts of matter, results were as in the following table oz.
With Water . . . , 12 Oil 18 Venice Turpentine 24 Tallow Candle . . 800 Rosin 850 Pitch . . . . 1400 In estimating the absolute cohesion of solid pieces of bodies, he applied weights, to separate them according to their length : his pieces of wood were long square parallelopipedons, each side of which was 26 of an inch, and they were drawn asunder by the following weights : lb.
Fir 600 Elm 950 Alder 1000 Linden tree 1000 Oak 1150 Beech . 1250 Ash 1250 Ile tried also wires of metal, 1-10th of a Rhinland inch in diameter : the metals and weights are as follow : lb.
Of Lead . . • . 29+ Tin 40+ Copper 2991 Yellow brass . . 360 Silver 370 Iron 450 Gold 500 lie then tried the relative cohesion, or the force with which bodies resist an ac tion applied to them in a direction per pendicular to their length. For this pur pose he fixed pieces of wood by one end into a square hole in a metal plate, and hung weights towards the other end till they broke at the hole : the weights and distances from the hole are exhibited in the following table : Distance. Weight.
inc. oz.
Pine . . • 9 . .
Fir . . . • . 9 . . 40 Beech . . . 7 . . 56} Elm . . . . 9 . . 44 Oak . . . . 85 . . 48 Alder . . • . . . 48 See his " Elem. Nat. Philos."